Mixing in a co-flow-focusing structured droplet-based micromixer

The droplet-based micromixing technology has been widely used in chemical synthesis, biomonitoring, and other fields due to its good mass transfer performance. Compared with plug-shaped droplets, spherical droplets have higher productivity and better mass transfer performance. However, the traditional mixing intensification method cannot effectively break the vortex symmetry inside the droplets at the changes of the channel structure, limiting the mixing performance of spherical droplets. This study proposed a co-flow-focusing structure to realize mixing enhancement by changing the initial distribution of internal components of newly formed droplets. A full-cycle multiphysics field model from droplet generation to mixing and exiting was developed to further reveal the evolution of droplet mixing dynamics based on internal vortices. The effects of the dispersed phase flow rate Qd, the continuous phase flow rate Qc, and the local geometry on the mixing performance were investigated. The results show that high initial mixing efficiency and internal vortex strength both can enhance mixing. There exists a critical dispersed phase flow rate Qd* leading to the lowest mixing rate. Conversely, increasing Qc enhances the initial mixing efficiency and internal vortex strength. The final mixing efficiency within the droplet (t = 50 ms) was increased by 15 % when Qc was varied from 2.20 μL/min to 5.04 μL/min. Additionally, the contraction orifice further enhances the mixing performance of the co-flow-focusing structure. The structure proposed in this paper simplifies the design of droplet-based micromixers, and the findings contribute to the further development of the co-flow-focusing structured droplet-based micromixer.